revert changes to `gratipay/security/csrf.py`
revert changes to `gratipay/security/csrf.py`

gratipay/gratipay.com,studio666/gratipay.com,gratipay/gratipay.com,gratipay/gratipay.com,studio666/gratipay.com,mccolgst/www.gittip.com,eXcomm/gratipay.com,eXcomm/gratipay.com,mccolgst/www.gittip.com,studio666/gratipay.com,eXcomm/gratipay.com,mccolgst/www.gittip.com,studio666/gratipay.com,mccolgst/www.gittip.com,eXcomm/gratipay.com,gratipay/gratipay.com